Electric generators used for electric power generation include a rotor and a stator having a stator core. The stator core is fabricated from thin laminations and includes a plurality of stator slots each adapted to receive an associated electrical winding formed as a coil. Each coil is retained within its corresponding slot by a wedge device for providing a tight fit so that movement of the coil relative to the stator core is minimized. A global vacuum pressure impregnation (GVPI) process is then used to impregnate the entire stator with an epoxy based resin coating. Such coatings serve to bond the coils to the stator to further minimize relative movement of the coils while also providing electrical insulation, corrosion resistance and other benefits.
A significant number of electric generators currently in use have been manufactured using the GVPI process and have been in operation for several years. However, it has been found that the coating breaks down and erodes over time thus requiring that the stator be repaired. An option for repairing the stator is to perform a field rewind of the stator at a customer location. During a field rewind, the wedges and stator coils are removed but a significant portion of the original coating remains. Prior to the installation of replacement coils and wedges, the remaining coating must be removed from the stator slots in order to obtain proper coil fitment and electrical contact between the new coils and the stator core. The remaining coating is approximately 0.020 inches thick and is comprised of layers of resin and mica insulation and is difficult to remove from the slots.
A method used to remove coating from the stator slots is to manually remove the coating using hand held brass scrapers. However, this process is inefficient, labor intensive, and may damage the stator core iron if performed incorrectly.
Another method utilizes high pressure water to remove the coating from the slots. However, it has been found that high pressure water undesirably penetrates between the core laminations and results in the removal of insulation between the laminations. This causes undesirable electrical shorts between individual laminations. Further, the water oxidizes the iron in the stator core and causes rust on the laminations.